The Concept
A photo image can be considered, from a photogrammetric (photogrammetry) point of view, a central perspective of the acquired object with good approximation. If the internal and external orientation of the camera are known, in case of a digital image it is possible to establish the direction in the space of each object point represented by a pixel in the image. If only one image is available, it is impossible to determine the spatial X,Y,Z position of such object points, because the simple direction is insufficient: all the points along that direction would give the same image point. By means of a DDEM (Dense Digital elevation model) of the acquired object, every pixel (and therefore every direction in the space)can be associated to the value of distance between the centre of perspective and the object point represented by the pixel itself. In this way each pixel can be referred to the 3D position of the corresponding object point in an absolute reference system. The DDEM (Dense Digital elevation model) can be derived from an existing map or by the use of surveying instruments and procedures. Recently, a series of new instruments, based on the laser technology (3D scanner), have been introduced on the market, giving the possibility to obtain a DDEM in a quick and cheap way.
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